Usually a coordinate measuring machine (short CMM) has a tactile probe or an optical probe for gauging the surface of a target object. The optical or tactile probe is movable fixed at an articulated arm, as it is shown for a tactile probe i.e. in EP 2283311 A1, or at a portal, as it is shown for an optical probe i.e. in WO 2008/135530 A1, so that it can be moved over the surface of the target object. In order to prepare such a measurement the CMM usually has to be calibrated by means of a reference object and a program has to be written defining the trajectories for the measurement probes. Immediately before a measurement can be started it has to be checked whether the target object is fixed properly in the correct position and whether there are any obstacles in pathways of the measurement probe. That means that beside the measurement itself a time consuming calibration has to be done before measuring and only specially trained users can carry out the measuring.
In industry measurement times are considered as unproductive times as no salable components are produced during this period. The measurement task therefore has to be done as quickly as possible. Thus, high measurement speed and short preparation times, including a quick fixation of the object to be gauged and a short calibration time for the CMM, is of high commercial importance. In this context it is understandable that not only the maximum moving speed of the measurement probe relative to the target object is of interest, which is mainly important for large components and long distances to be traveled by the measurement probe, but also the maximum acceleration and deceleration, which is important for small work pieces as it allows a very fast positioning of the measurement probe on the interesting positions relative to the target object.
Therefore, several measures have been taken during the past to increase the measurement speed. E.g. measuring with an optical probe instead of a tactile probe in general can increase the measurement rate and avoid abrasion effects at the surface of the target object.
Another option to increase measurement rate is the use of a camera as a measurement probe and using this camera in an “on the fly”-mode, as it is described in WO 2008/135530 A1. During the “on the fly”-measurement mode the camera is moved continuously over the target object and takes pictures only at the interesting positions without stopping there. The position data for each image is delivered from the position encoders and stored together with the according image. A flash light illumination of the interesting positions ensures a sharp picture in spite of the moving speed of the camera. As the camera is not stopped at the interesting positions, less deceleration and acceleration actions have to be carried out which decreases measuring time. However, in order to know the interesting positions and in order to move the camera along optimized trajectories including all interesting positions calibration of the CMM usually on the basis of a reference object is necessary and a subsequent programming of the trajectories including the defined interesting positions, where pictures should be taken.
However, using a normal, cost-efficient camera with a standard sized optics shows a small field of view, when used with the magnification adequate to reach the necessary accuracy. As the field of view is small, it is necessary to take a lot of images, which means a lot of movements of the camera in order to see all features of interest. Thus, the throughput of a commercial CMM with an articulated arm or a portal structure—independent of using it with or without an “on the fly”-mode—is still non-satisfying caused by its low speed and low acceleration. In order to encounter this unsatisfying situation nowadays many CMMs are offered with cameras provided with objectives of larger diameter, showing a larger field of view for the same magnification. As a result no movements are necessary to measure small target objects and only a few movements and a few images are necessary for encompassing a large target object. However, those CMMs are expensive, as the price for cameras with such a large objective is high.
The latest development tries to increase the measurement rate by using a so called Delta Robot instead of a portal machine or an articulated arm for moving a tactile measurement probe (brochure “Equator 300 Mess-Systeme” of Renishaw, published in July 2011).
A Delta Robot is a type of parallel robot. It comprises a stationary base fixed at a stationary frame, which is mounted above a workspace, and three middle jointed arms extending from the base. The arms, often called kinematic chains, are connected with their first end to the base by means of universal joints and connected with their second end to an end effector often built in form of a triangular or circular platform. The arms are made of lightweight composite material and are driven by actuators located in the base. Driven by the actuators the end effector is movable within a motion zone. The motion zone is the 3-dimensional space the end effector is maximally movable in. The boundaries of the maximum movement—and by that of the motion zone—are defined by the construction of the cinematic chains and the resulting physically limits of their common motion as being linked by the end effector. Actuation can be done with linear or rotational actuators. As the arms are made of a light composite material the moving parts of the delta robot have a small inertia. This allows for very high accelerations and very fast movement, which outclasses by far those realizable by a portal machine or an articulated arm. The key design feature of a Delta Robot is the use of parallelograms in the arms, which maintains the orientation of the end effector by restricting the movement of the end effector to pure translation (movement only with 3 degrees of freedom (3DOF: translation in the x-, y- or z-direction). Nowadays in industry Delta Robots are mainly used for gripping and placing items, wherein the gripper is arranged at the end effector. The movement of the end effector and precise gripping and placing of items is controlled by a main controller getting feedback information of the actuators and of angle encoders connected to the joints of the arms often named position encoders. A trajectory of the end effector from a position an item has to be gripped to a position where the item has to be placed is stored in the main controller. During operation the main controller controls the actuators of the arms in a way that the end effector follows the programmed trajectory.
In a further development machines based on a Delta Robot structure had been created. Thereby the degree of freedom (DOE) of the Delta Robot had been extended up to 6, allowing the end effector lateral movements in Cartesian directions x, y, z and rotational movements around those axis resulting in yawing, rolling, pitching. Because of their high acceleration/deceleration actions and their high movement speed Delta Robots and machines based on a Delta Robot are popular for picking and packaging in factories of the packaging industry, medical and pharmaceutical industry; some executing up to 300 picks per minute. Other possible applications include assembly tasks or operation in clean rooms for electronic components as well as haptic interfaces controlling medical or industrial robotic systems for enabling human operators to operate instinctively and safely critical systems.
But in spite of its applicability in various technical fields, Delta Robots have been found so far not suitable for measurement requirements. This is because of their sensitivity to temperature fluctuation and strong vibration during fast movement and fast acceleration/deceleration actions, caused by their lightweight construction. As a result, the exact position of the end effector can not be determined precisely enough and adequate focusing with optical means, e.g. an optical probe for a CMM or a camera, is not possible.
The usage of a Delta Robot for moving a tactile probe of a CMM, as it is proposed in the brochure “Equator 300 Mess-Systeme” of Renishaw, is therefore not in contradiction to the statement above, but considers these problems of Delta Robots by using a tactile measurement probe. As the tactile probe has to contact the surface of the target object, the tactile probe dictates the measurement speed, which is much slower than what the Delta Robot would allow. Thus, the measurement rate is limited by the tactile probe anyway and determination of its position will thus be able. However, the possibilities the delta structure provides with respect to acceleration and motion speed is not fully exploited. Further, the CMM disclosed in this brochure is again only be able to measure the target object with reference to a reference object, which has to be measured before the target object of a series production can be measured. That means it has to be calibrated before the measurement. So, no cut down of measurement time is possible during preparation of the measurement and only special trained persons will be able to handle this CMM.